Low capacitance power supply



Oct. 4, 1960 J. H. REAvEs LOW CAPACITANCE POWER SUPPLY 955 2,955,246 original Filed May 9, 1 2 sheets-snaai 2 Vhas been lreduced to an extremely low value. .of such feature it becomes possible -to construct various embodiments of a novel -direct-coupled amplifier employing the power supply according to this invention.'

United States Patent LOW CAPACITANCE POWER SUPPLY Iohn H. Reaves, McLean, Va., assignor to the United I States of America as represented by the Secretary of Commerce Original application May 9 1955 Ser. No. 507 186. Dif lvided and this application Nv. 29, 1957,ser. No.

2 Claims. (Cl. 321-8) -pending application Serial No..507,186 filed on May 9, '1955 for a Direct-Coupled Amplifier Construction. f' -Some direct-coupled amplifier circuits require the use of a direct-current power source which can neither be "Y grounded nor by-passed to ground if proper operation of the circuit is to be obtained. In such instances a conventional power supply cannot be used because of the large inherent shunting capacity of a conventional type of power transformer. Although batteries would be suitable in such application they suffer the disadvantages of 'short life, and"unadjusta'bility of voltages;

Conventional power transformers such as are employ in typical "power supplies" areY Ydesigned forY maximum -power transfer eiciency, and because 'of the required close 'proximity between the windings and core in order to secure maximum coupling, such construction results in characteristically high capacitance between the wind- -ings and between the windings and core. When it is attempted to employ a conventional power supply in a circuit in which the power supply cannot be grounded or by-passed to ground, the effect of the capacitance is to -shunt signals to ground. Moreover, even should a conventional power supply be employed in connection with such described type of circuit, signal losses would'occur for all but the lowest frequencies. The present invention overcomes such limitations through the use of a power supply circuit which employs a specially constructed power transformer in which the secondary capacitance Because A low capacity transformer construction for filament Asupply use is disclosed in U.S. Patent No. 2,214,083v 4by I. L.' Finch, but so far as is known no attempt has heretofore been made to employ a low-capacitance powerlsup- .ply for plate and bias voltage supply.

It istherefore an immediate object of this invention to provide a power supply, particularly adaptable to direct-coupled amplifier construction, which possesses a low shunt capacity and can therefore be used in circuits 'requiring the power supply to operate at a (signal) varying potential level with respect to ground.

. A further object of this invention is to provide a power supply employing a special transformer construction in which the secondary windings are capacitively separated lfrom the core and from the primary winding.

It is an additional object of this invention to provide a power supply which is particularly adaptable for use in direct-coupled circuit applications where neither terminal of the power supply can be grounded, or by-passed to A still further object of this invention is to provide a ice ponents can be physically mounted remote from those circuit elements which are at ground potential, and in proximity tothe secondary winding of the transformer so that the shunt capacitance of the entire secondary circuit is kept at a minimum value.

An additional object of this invention is to provide various embodiments of direct-coupled amplifier circuits in-which the voltage requirements are obtainable from a simple unitary type of A.C. operated power supply.

Other uses and advantages of the invention will be apparent uponreference to the specification and drawings in which:

Fig. 1A is an isometric view showing a structural embodiment of the present invention;

Fig'. 1B is an isometric view showing the transformer construction in greater detail; l

Fig. 2 is a circuit diagram of an electronically regulated power supply circuit used in connection with the present invention; l

Figs. 3A through 3C show various circuit modifications employing the power supply according to the present inventiori for interstagecoupling and as a pentode-cathode follower circuit having good high frequencyjrespons'e;

Fig. 4A shows the present invention applied to a voltage 'amplifier circuit suitable. for driving a capacitive load;

lFig. 4B shows a two-.tube cathode follower circuit employing the features of the present invention suitablel for driving a capacitiveload, and 1".-

Fig. 4C shows amodifcation of Fig. 4A employing a pentode.

`'Iheabo'v'e obiects are lachieved according to the teachings of the present invention by employing a specially constructed power transformer in which the'secondary winding is capacitively vseparated from the core, leaving as Ir'nu'ch air gap as the geometryof the construction permits. .The effect of such preferred construction is to lower the capacitance of the secondary winding of the .transformer-in respect yto the core and primary windings, respectively.

potential. Thecomponents are further mounted in a novel fashion adjacent to the transformer on an insulated chassis which; for all practical purposes, .is an integral ypart of the form which contains the secondary winding.

Such construction results in a'very low value to the stray or shunting capacitance of the entire secondary circuit.

Fig.1A shows a practical embodiment of a power'supply employing the featuresof this invention. As shown in Fig.' 1A 'thepower'supply includes a power transformer having a laminated core 1 of ay size and configuration commensurate with the voltage and power requirements.

The primary winding consists of a coil 2 having two sections. eachmounted on one leg of the transformer as shown in Fig. 1B. Only one section is shown in Fig. 1A. The coils are series connected and have external leads v3 3 adapte'd to be connected to a suitable 60cycle A.C. source. As indicated, the primary windings are each mounted on a respective spool-4 which is made of a suitable insulator such as Plexiglas, Lucite, etc., and are mounted on each leg of the core.

In order t0 reduce the capacitance effect of the secondary circuit of the-power supply to a minimum, the secondary winding 5 is physically mounted on a large spool 6, which, as shown, has a re-entrant portion which passes through the throat of the transformer core. The spool for the secondary winding is also made of an insulating ma- Fig. 1.

3' terial, such as Plexiglas, and is arranged relative to the transformer core as indicated in Fig. 1, so that the secondary winding 5 is separated from the core as well as from the primary windings by the maximum amount or air gap which the geometry of the construction permits. As is apparent from Figs. 1A and 1B, only the re-entrant portion of the spool need be in proximity to the core and primary winding in order to obtain magnetic coupling between the windings necessary for transformer action. It follows that any capacitive effects between the secondary and primary windings are substantially limited to the region involving the re-entrant portion of the secondary. The physical characteristics of the various components employed in a typical power transformer construction according to the present invention may be tabulated as follows:

Transformer core dimension:

Cross-section S x 1% inches. Outside dimension 3 x 3 x 1% inches. Primary windings:

Number of turns 2 coils, 500 turns each. Wire size No. 28. Spool dimensions:

Cross-section 1%@ x 11%; inches. Length 1%@ inches. Secondary windings:

Number of turns 5200 center-tapped. Wire size No. 34. Spool dimensions:

Cross-section 2% x 1% inches. Length A inch.

The core of the transformer is constructed of transformer steel, the lamination thickness being approximately 0.017 inch, a value suitable for 60-cycle operation and which value may be decreased for higher frequencies of operation. Enameled Wire is preferred for all windings because of its compactness, vand the primary consists of two aiding series-connected coils mounted on opposite legs of the square periphery core 1. Such construction reduces magnetic leakage while permitting a maximum amount 0f clearance for the secondary winding. A transformer construction in accordance with the above specifications is capable of delivering approximately 30 milliamperes at 160 volts while manifesting only 18 micromicrofarads capacitance to ground. It is to be noted that the shunt capacitance of a conventional comparable power supply generally is approximately 700 micromicrofarads.

The above-featured power transformer construction further permits the mounting of the rectifying, filtering, and voltage regulating elements of the power supply in a novel manner, a construction which results in a very low value to the stray or shunting capacitance of the entire secondary circuit.

The physical arrangement of the rectifying, filtering, and voltage regulating components of a typical power supply according to this invention is showin in Fig. 1A. As indicated, all of such elements are mounted on an insulated platform comprising a mounting panel 11 provided with a pair of transverse mounting flanges 12-12. The material of the panel and flanges may be any convenient insulator, such as Plexiglas, Lucite, etc. The power transformer described includes a plurality of mounting studs 13 to which flanges are secured. All of the electronic components comprising the rectifying, filtering and voltage regulating elements are compactly mounted on the panel 11 as indicated.

Fig. 2 shows the complete circuit diagram for the power supply, the physical embodiment of which is illustrated in The various components mounted on the panel as illustrated in Fig. 1 may readily be identified, and their circuit relationship established in connection with the diagram of Fig. 2. f

Specifically, as the circuit diagram indicates, a rectifier V-1 of the 6X4 variety, for example, is mounted in a suitable socket '14 secured directly to the panel 11. The tubes V-Z, V-3, and V4 which are of the type indicated in Fig. 2, comprise the voltage-regulator circuit of the power supply and are mounted in suitable sockets 15, 16, and 17 in convenient relationship to the rectifier tube on the panel 11. The three-section filter condenser C-18 and the voltage control potentiometer 19 are also secured to the mounting panel 1-1.

Returning to the description of the power transformer, it will be noted from Fig. 2 that the secondary winding 5 is designed to provide a plurality of output voltages. The three heater windings 5a, 5b, and 5c in the transformer may be mounted on top or underneath the high voltage secondary winding. The remainder of the circuit elements comprising the power supply are arranged on the lower surface of the mounting panel 11 in accordance with the wiring diagram of Fig. 2. Suitable output terminals 20 are provided on the mounting panel 11.

The electrical configuration of the typical regulated power supply circuit employed in the present invention is conventional except that it is ungrounded and will therefore not be described in detail. A general description of an equivalent grounded supply is given on pages 378- 379 of Electronics Experimental Techniques by Elmore and Sands.

The isolation of the secondary winding causes the eiciency' of the transformer to be lowered somewhat, partly because of the decrease in the resulting magnetic copling, and partly because of the increased circumference of the windings. Although for the low-power applications contemplated in the subsequent description, poor power efficiency is not in itself serious, the accompanying increase in percentage voltage drop under load (regulation) is important. However, by employing electronic stabilization in the power supply circuit, as shown in Fig. 2, good regulation of the D.C.-output voltage can be achieved in spite of the relatively poor regulation of the transformer.

The power supply constructed according to the specification described provides full wave rectification and with the following regulation characteristics:

No load voltage: Maximum lo'ad voltage 156 at 30 ma.

200 199 at 20 ma. Shunt capacitance 18 auf. Figure of merit (see below) 260.

parison, the shunt capacitance of a typical conventional supply for the same power measured approximately 700 tmf. Much of this ditferenceis attributable to the special transformer construction described.

In order to determine the relative merits of the power supplies constructed in accordance with the teachings o'f this invention, a suitable figure of merit was assumed to be the ratio of the maximum power output in milliwatts to the total shunt capacitance in auf.

Several power supplies employing the special 10W- capacitance type of construction described were constructed for various current and voltage outputs as is indicated in Figs. 4A and 4B which show various exemplary voltage requirements.

The usefulness of the low-capacitance type of power supply can be demonstrated by several examples of novel circuits which can be constructed in connection with such type of power supply.

A novel direct-coupled amplifier construction employing the power supply of the present invention is shown in Figs. 3A and 3B, the power supply being used in two different ways to provide direct interstage coupling. The

output terminals A and B of the power supply circuit in yU.S. patent`Serial No. 2,358,428 issued to E. L. C. shown in Fig. 2 are similarly identified in the various White, and P. `G.`Sulzer in an article entitled Survey embodiments illustrated in Figs. 3A3C and 4A-4C. of Audio Frequency Power-Amplifier Circuits, appearing Referring to Fig. 3A, the power supply employed is in Audio Engineering, vol. 35, No. 5, May 1951, the

ungrounded and has a very low secondary capacitance, 5 particular two-tube direct-coupled circuits described in therefore, signal attenuation due to the referred to shunt connection with Figs. 4A and 4B are believed to be novel capacity elfects is extremely small even for very high because of the singular relationship between the circuit signal frequencies. In the particular embodiment of the and the power supply employed.

invention shown in Fig. 3A, the power supply is not re- The amplifier circuit of Fig. 4A has, in addition td low quired to supply any current. output impedance, good linear-ity and a voltage gain very The modilication of Fig. 3B shows the power supply nearly equal to the amplification factor of stage V-41. providing the plate Current for Stage V-`While Serving An improved version of this circuit is obtained by using a as the coupling means between stages. In this modificapentode in place of a triode for the tube V-40 as shown tion, the magnitude of the plate voltage is not critical in Fig. 4C. In such case, the power supply is not needed especially if a pentode-type tube is used for the amplil5 in the position shown in Fig. 4A but instead is connected iier V-30 in which case the screen would be held at a as in Fig. 3C so as to supply the screen-grid pottential constant potential with respect to cathode and ground. in the manner illustrated in Fig. 4C. The amplifying cirin Fig. 3B there is also symbolically illustrated in dotted cuit of Fig. 4A can be modified by adding a capacitor lines the capacitive shuntng effect (C-30) imposed on 41 .from the plate V-41 to ground thus making a novel the circuit when a conventional power supply is used. 20 linear sawtooth generator of the familiar bootstrap type.

The advantages of an amplifier construction embody- The two-tube cathode follower circuit of Fig. 4B has ing the principles of the present invention can best be higher input impedance, better linearity, nearer unity appreciated by comparison with a conventional directgain, and lower output impedance than a conventional Coupled amplifier Sueh as is described on pages 529-531 single tube cathode follower. As an example of its high (see Fig. 12.44) of the Radiotro'n Designers Handbook. 25 efiiciency when driving a capacitive load, this circuit with ile, in such type of construction, the shunt-capacity the component values shown, will supply peak charge and eifects of the power supply is not important because it discharge current of 80 ma. with an average plate circuit is already grounded, there exists the disadvantages Of current as low as 15 ma. Aconventionalsingle-tube circuit (l) signal attentuation by the voltage divider action giving comparable square-wave output with the same load across the resistors R-Z and R-3, and (2) the VOltage 30 capacitance requires an average plate current of approxidivider circuit has to be capacitively compensated by the mately 100 ma.

inclusion of a capacitor across the resistor R-Z in order The above exemplary embodiments typify instances in to compensate for tbe shuntng eieet occasioned by the which direct-coupled circuits must be supplied with a tubeS input across the resistor R3. In conventional D C. source which cannot be grounded or bypassed to direct-coupled amplifier circuits such as are represented ground By combining the novel low-capacitance power in the referred to teXt such compensation iS Critical and supply with such direct-coupled circuits in accordance sometimes unsatisfactory due t0 VariatOnS in the input with the teachings of this invention it becomes possible impedance to tbe tube- Moreoyer, aS Compared to a to construct such circuits for convenient A.C. operation conventional direct-coupled amplifier circuit construction, and thereby eliminate the need for batteries. such as describedin Fig. 12.44 of the referred to Hand- While a preferred embodiment of the invention haS book, the circuit shown in Fig 3B is relatively insensi been disclosed and described in the accompanying drawtlVe to Variations 1n tbe Value of tbe supply Voltage ingss 1t w1ll be apparent that vanous modifications and The Power supply is shown in Fig- 3C as being ein embodiments of the singular features of the invention deployed i0 Proyide screen-grid potential in a pentodecaihscribed would readily follow from the present disclosure.

ode follower crcuit- As shown terminil1 A of the de* 45 For example, in the construction of the power transscribed power supply is connected to the screen grid of former, the primary windings could be capacitively isoa pentode V32 While terminal B is connected to the lated and separated from the secondary, while to further tubgo catllode- The output is obtained across resistor reduce the capacitive effects both the primary and sec- 50 ondary of the transformer could be physically separated TWO duwt-coupled clrcmts m which the novel Power with respect to the core. It is therefore not intended to supply has been found to be Particularly useful are illus' restrict the present invention to any specific construction trated in Figs. 4A and 4B. The two-tube amplifier illusexcept as defined inthe appended claims.

trated in Fig. 4A employs a pair of tubes V40, V41 Whatis elaimedis;

of the 6AF4 type- Terminal B of the PoWer supply is l. A low capacitance A.C. operated direct-current Shown Connected to the Cathode of tube V40 While ter 55 power supply suitable for use in direct-coupled ampli- In-inal A is connected through a resistor R40 of the plate fier applications for supplying D.C. potentials to selected of tube V41- The input signal is applied across the elements of the amplifier stages and for interconnecting grid or tube V41 and the output iS obtained across the said stages comprising, a power transformer having an catllodes of tubes V40 and V-ll respectively as indi* open throat magnetic core, a primary winding wound catedclosely on said core, and a secondary winding compris- A tWo'tu'be cathode follower circuit employing the ing an annular spool having a sectorial portion physicalpower Supply aeccrdlng t0 the present invention is Shown 1y contained within said core, the remaining portion of in Fig 4B- In this Circuit the tubes V-42 V-43 may said spool being substantially spaced from said core and be of the 5cl-5 type and the power supply iS connected primary winding so as to leave the maximum air gap as indicated to the Plate of V43 and to the Cathode 0f 65 therebetween within the limits defined by the geometry of V42, reSPeCtiVely- The input Signal iS applied t0' tbe the physical components employed, means for converting grid of V-42 and the output is obtained across the cirthe alternating current in said secondary winding into cuit including the power Supply and tube V-43 in SeIieS direct current, means mounting said converting means in the position of maximum physical proximity to said shown in Figs- 4A and 4B giVeS better high-frequency tion of the components comprising said converting means response for a given average plate current than a conand remotely with respect to said core and primary windventional single-tube amplifier or cathode follower. ing, said converting means comprising a full wave recti- While circuits of this general type, employing twd seriesfier tube, a voltage regulator tube, and electronic load connected tubes driven in opposite phase, are described regulating tube means operatively associated with said 2,955,246 regulator tube, said 4secondary including heater windings References Cited in the file of this patent for each of said tubes, said heater windings being mounted UNITED STATES PATENTS Contiguous with said'secondary winding.

2. The invention as defined in lclaim 1 in' which said 2,021,380 Raskhodoff Nov. 19, 1935 secondary winding and said heater windings are wound 5 2,301,343 Tal-r No 10, 1942 on an insulated spool only a portion of which is mounted 2,844,761 Nieswaag et a1. July 22, 1958 in proximity to said core and to sai-d primary windings and in which said components comprising the converting OTHER REFERENCES meals are .momtedn an inild platform; Secure Low Capacity Transformers for Grounded Grid Apto t e portion o sai spool w 1c 1s remote rom sai 10 plicatonsl, (C B' Adelmann), CQ Radio Amateur,s

core and said primary windings. Journal (July, 1956), pages 40-42 relied on. 

